The present invention relates to methods of fabricating or manufacturing mufflers, and more particularly, mufflers for vehicular exhaust systems used with internal combustion engines.
Over the years, vehicular exhaust system mufflers, silencers, resonators and the like (hereinafter collectively referred to as "mufflers") have employed many different designs. Nevertheless, most mufflers have included one or more end tubes; one or more flow tubes that may be perforated, in whole or in part, so as to permit the expansion of the combustion gases within the muffler; one or more panels or baffles that are often described as end panels and internal panels, depending on their dispositions within the muffler, and that among other things serve to support the tubes; and an inner, and often an outer sheet metal body wrapper that serves as a housing for the other muffler components. Generally speaking, the fabrication of mufflers in the past has involved a number of steps, including the assembly of a reversing unit and end cap sub-assemblies.
In one muffler design, the reversing unit sub-assembly includes two internal panels and two flow tubes. The peripheral edge of each internal panel has an integral flange that projects perpendicularly from the plane of the panel. Each of the internal panels also includes one or more apertures, and each aperture is surrounded by an integral, perpendicularly projecting flange. The internal panels are disposed, side by side, so that in general, apertures in the panels are aligned in pairs.
In fabricating the reversing unit sub-assembly, the opposite end of the flow tubes are secured within aligned apertures by a mechanical lock joint, such as a pair of continuous circumferential beads or a continuous circumferential bead and flaring the end of the tube. The inner sheet metal wrapper is folded or configured into a generally oval or round shape so as to define a generally cylindrical volume into which the reversing unit sub-assembly may be inserted. The ends of the wrapper are joined together by seam locking, spot welding, seam welding or the like. One of the internal panels is inserted into the inner wrapper first and is called the leading panel. The internal panel, inserted into the inner wrapper last, is called the trailing panel.
At this point in the muffler fabrication, it has been customary to make a number of spot or tack welds between the inner wrapper and the peripheral flange on the trailing panel. These welds attach the trailing panel to the inner wrapper and thus locate the entire reversing unit sub-assembly relative to the inner wrapper.
The next step in the fabrication has been to apply an outer sheet metal wrapper around the assembled reversing unit sub-assembly and inner wrapper. The outer wrapper is shaped or configured about the inner wrapper such that the respective side edges of the two wrappers are adjacent. Its end edges are then seam locked together so as to hold the outer wrapper tightly in place over and around the inner wrapper.
Two end cap sub-assemblies are usually required to complete the fabrication of the muffler. Each end cap sub-assembly includes an end panel having a continuous peripheral edge. Each end panel has at least one aperture that is surrounded by a perpendicularly projecting, aperture surrounding flange. Each of the end tubes is disposed, between their ends, in its end panel aperture and is secured to the end panel by a mechanical lock joint such as shown in U.S. Pat. No. 4,565,260.
In the assembled muffler, an end of each end tube is usually connected with an end of a flow tube. In the past, the connected end of the end tube is sized so as it can be tightly press fit within the inside diameter of its associated flow tube end when the end cap assembly is brought adjacent to the side edge of the inner and outer wrappers. This press fitting requires the maintenance of close or narrow tolerances. The overlapped, press fitted ends of these tubes have been secured together by a mechanical lock joint such as shown in U.S. Pat. No. 4,570,747. The press fitting poses no problem with respect to dislocating the reversing unit sub-assembly, relative to the inner and outer wrappers because as noted, that sub-assembly is held in place within the wrappers by the plurality of spot or tack welds.
After the end tubes have thus been connected with the flow tubes, the side edges of the wrappers and the peripheral edges of the end panels are spun and crimped together. This completes the fabrication of the muffler.
While the above method of fabricating mufflers continues to produce a quality muffler product, the necessity of utilizing welds to hold the reversing unit sub-assembly, relative to the inner wrapper, is a time consuming and thus expensive step. Additionally such welding can pose potentially serious health and workplace safety concerns for the fabrication workers. Further such welding can adversely affect the integrity of the protective coating applied to the muffler components by creating a site of nucleation of corrosion. This defeats the purpose of applying the protective coating and of using mechanical lock joints. Those working in this art have long sought to eliminate having to weld during muffler fabrication.
The above-mentioned use of mechanical lock joints has reduced the use of welding--and its resultant disadvantages--in muffler fabrication. Nevertheless, to obtain sufficient torsional resistance for such a mechanical lock joint, such as, for example, the joint shown in U.S. Pat. No. 4,565,260, a die has to be used as a backup for the rotary tooling utilized to deform the tube surface into the joint. Moreover, tooling such as disclosed in U.S. Pat. No. 4,821,391, is required in order to form such mechanical lock joints on a production line basis. The cost of such tooling is relatively expensive, and the formation of the joints, even using such sophisticated tooling, is relatively time consuming. Thus, the art has also long been seeking improved mechanical lock joints that can secure against the torsional and axial forces normally experienced in mufflers during their assembly and that can be relatively quickly and inexpensively formed.